1. Field of the Invention
The present invention relates to a charge/discharge control device and a power generation system, and more particularly, it relates to a charge/discharge control device employed in a power generation system including a power generator generating power with natural energy and a storage portion capable of storing power and a power generation system including a charge/discharge control device.
2. Description of the Background Art
In recent years, the number of cases in which power generators (solar cell etc.) utilizing natural energy such as wind power or sunlight is provided in consumer homes (consumer homes, factories, etc., for example) receiving AC power supply from an electricity substation has increased. Such power generators are connected to a power grid provided under the substation, and power generated by the power generators is output to in-consumer power consuming devices. Surplus power not consumed by the in-consumer power consuming devices is output to the power grid. The flow of this power from the consumer to the power grid is referred to as “reverse power flow”, and power output from the consumer to the power grid is referred to as “reverse flow power”.
The obligation to supply power stably is imposed on power suppliers such as electric power companies, and the power suppliers need to maintain the stability of the frequency and voltage of the overall power grid, including reverse flow power components. For example, the power suppliers maintain the stability of the frequency of the overall power grid by a plurality of control methods in correspondence with the size of a fluctuation period. Specifically, in general, in respect of a load component with a fluctuation period of at least about 20 minutes, economic dispatching control (EDC) is performed to enable output sharing of power generated in the most economical manner. This EDC is based on the daily load fluctuation expectation, and is difficult to respond to the increases and decreases in the load fluctuation from minute to minute and second to second (the components of the fluctuation period smaller than about 20 minutes). In that instance, the power companies adjust the amount of power supplied to the power grid in correspondence with the minute fluctuations in the load, and perform a plurality of controls in order to stabilize the frequency. These controls other than the EDC are called frequency controls, in particular, and the adjustments of the load fluctuation components incapable of being adjusted by the EDC are performed by these frequency controls.
More specifically, for the components with a fluctuation period of not more than about 10 seconds, their absorption is enabled naturally by self-regulating characteristics of the power grid itself. Moreover, for the components with a fluctuation period of about 10 seconds to several minutes, they can be dealt with by the governor-free operation of the power generators in each generating station. Furthermore, for the components with a fluctuation period of several minutes to about 20 minutes, they can be dealt with by load frequency control (LFC). In this load frequency control, the frequency control is performed by the adjustment of the generated power output of a generating station for LFC by means of a control signal from the central power supply command station of the power supplier.
However, the output of power generators utilizing natural energy may change sharply in correspondence with the weather and the like. This sharp change in the power output of this type of power generators has adverse effects on the degree of stability of the frequency of the linked power grid. These adverse effects become more pronounced as the number of consumers with power generators utilizing natural energy increases. As a result, in the event that the number of consumers with power generators utilizing natural energy increases even further henceforth, there will be a need arising for sustenance of the stability of the power grid by the control of the sharp change in the output of the power generators.
Therefore, in order to suppress this sharp change in the output of the power generators, a power generation system including a power generator utilizing natural energy and a storage portion capable of storing power generated by the power generator is proposed in general. Such a power generation system is disclosed in Japanese Patent Laying-Open No. 2007-228737, for example.
The aforementioned Japanese Patent Laying-Open No. 2007-228737 discloses a power generation system including a solar cell, an inverter connected to the solar cell and a power grid, and a charge/discharge portion connected to a bus connecting the inverter and the solar cell, and a storage portion connected to the charge/discharge portion. In the aforementioned Japanese Patent Laying-Open No. 2007-228737, the charge/discharge portion is controlled to perform charge/discharge of the storage portion following fluctuation in power generated by the solar cell, whereby fluctuation in power output from the inverter is suppressed. Thus, fluctuation in power output to the power gird can be suppressed and hence adverse effects on the frequency or the like of the power grid can be suppressed.
However, in the aforementioned Japanese Patent Laying-Open No. 2007-228737, the charge/discharge of the storage portion is performed each time following the fluctuation in the power generated by the power generator, and hence the charging/discharging count is increased so that the lifetime of the storage portion including a secondary cell or the like is disadvantageously decreased.
The present invention has been proposed in order to solve the aforementioned problem, and an object of the present invention is to provide a charge/discharge control device and a power generation system each capable of increasing the lifetime of a storage portion while suppressing effects of fluctuation in power generated by a power generator on a power grid.